The development of the gas turbine engine into the fuel-efficient, durable, high-powered propulsion engine as used on today's aircraft has depended to a large extent on the development of high strength nickel-based superalloys for the fabrication of hot-section turbine components. Such superalloys whether they be polycrystalline, directionally solidified or monocrystalline exhibit creep, stress-rupture and tensile strength properties superior to those of the earlier generation of nickel-based alloys. However, these superalloys generally have very poor ductility, and are difficult to cast or fabricate into engine components.
Production of turbine engine hot-section components from the new generation of superalloys is characterized by low yield and gross inefficiencies in the casting/fabrication process, thereby creating high part prices accompanied by routine shortages. In past years when turbine operators had been confronted with this dilemma in the operation and maintenance of their equipment, operators would be able to seek economy and parts supply through repair of the components. This approach was very successful until the advent of the new class of superalloys and their inherent high strength, low ductility properties which confounded the existing repair and restoration schemes. When castings or engine-run parts of these new superalloys are welded, cracks are induced which propagate rapidly under stress. These superalloys are principally strengthened through controlled heat treatment producing Ni.sub.3 Al or Ni.sub.3 Ti precipitates known as gamma-prime. The precipitation hardening phenomena and the associated volumetric changes that occur upon aging facilitates cracking and makes welding of these alloys very difficult.
Upon welding, a portion of the heat affected zone is heated into the precipitation hardening temperature range and undergoes a volumetric contraction resulting in residual stress in the weldment upon solidification, accompanied by a loss in ductility. Rapid heat-up and cool-down from welding temperatures produces complicated thermal expansion and contraction, leading to additional residual stress. These thermal stresses, when combined with previous stresses produced from the aging reaction, can result in cracking. This cracking, or fissuring, is often located in the heat affected zone. The heat affected zone is also subject to grain growth or even localized melting that makes the weld zone even more susceptible to cracking. Post weld solution annealing and/or aging heat treatments can further increase susceptibility to cracking.
Although substantial progress in brazing technology has been achieved, no substitute for the weld repair of cracks in highly stressed structural details or sealing surfaces has been discovered.